I wrote one! Yeah, not really. Someone else wrote a sample game where you walk around (no shooting yet) in plain HTML! That's right, this is from the HTML 5 that is being worked on 
http://canvex.lazyilluminati.com/
http://www.whatwg.org/specs/web-apps/current-work/
It uses the new <canvas> tag
http://canvex.lazyilluminati.com/
http://www.whatwg.org/specs/web-apps/current-work/
It uses the new <canvas> tag